The following abbreviations which may be found in the specification and/or the drawing figures are defined as follows:
AN Access Network
ARP Allocation and Retention Policy
BTS Base Transceiver Station
CS Circuit Switch
CSFB Circuit Switch Fallback
E-UTRAN Evolved Universal Terrestrial Radio Access Network
ECM EPS Connection Management
EDGE Enhanced Data Rates for Global Evolution
EMM EPS Mobility Management
eNode B Evolved Node B
EPC Evolved Packet Core Network
EPS Evolved Packet System
ESM EPS Session Management
GERAN GSM-Enhanced Data Rates for Global Evolution (EDGE) RAN
GBR Guaranteed Bit Rate
GSM Global System for Mobile Communications
IMS IP multimedia subsystem
LA Location Area
L-GW Local PDN Gateway
LIPA Local IP Access
MBR Maximum Bit Rate
M-TMSI M-Temporary Mobile Subscriber Identity
MME Mobility Management Entity
NodeB Node B
PCRF policy and charging rules function
PDB Packet Delay Budget
PDCP Packet Data Convergence Protocol
PDN-GW Packet Data Network Gateway
PEER packet error loss rate
QCI QoS Class Identifier
QoS Quality of Service
RAN Radio Access Network
RLC Radio Link Control
RNC Radio Network Controller
RRC Radio Resource Control
SDU Service Data Unit
S-TMSI S-Temporary Mobile Subscriber Identity
SAE System Architecture Evolution
SRVCC Single Radio Voice Call Continuity
TA Tracking Area
TAC Tracking Area Code
TAI Tracking Area identity
TAU Tracking Area Update
Long Term Evolution (LTE) and Long Term Evolution-Advanced (LTE-A), as specified in the 3rd Generation Partnership Project (3GPP), are all-Internet protocol (IP) technologies which are among the ongoing enhancements to the Universal Mobile Telecommunications System (UMTS) referred to as the Evolved UNITS Terrestrial Radio Access Network (E-UTRAN). Voice service in E-UTRAN is provided by way of voice-over-IP employing an IP multimedia subsystem (IMS) (referred to as “voice-over-LTE” (VoLTE)). Migration from legacy networks, such as 2G/3G/1xRTT networks employing circuit switch (CS) voice service, to a full-blown VoLTE employing packet switch (PS) voice service is not likely to occur in the near future. One interim solution is to utilise the circuit switch fall back (CSFB) mechanism which was introduced in 3GPP Rel-8. Another interim solution employs a single radio voice call continuity mechanism which was subsequently introduced in 3GPP Rel-9.
CSFB allows a user equipment (UE) registered with a public land mobile network (PLMN) to access (E-UTRAN) via the evolved packet system (EPS) to reuse CS domain services (i.e. voice services). This is applicable for example in a situation where the E-UTRAN access network does not support IMS services. In such a situation, where the UE is CSFB capable and the core network (CN) supports this functionality, the UE can switch its radio access technology (RAT) from E-UTRAN access to legacy RATs (e.g. GERAN/UTRAN/1xRTT access) which support CS domain services. Under certain circumstances, the UE employing the CSFB mechanism can maintain both voice services and data services simultaneously (including short messaging service (SMS). In certain 3G (UTRAN) scenarios, however, there would be a significant downgrade of any LTE data sessions. In a scenario involving fall back of the UE to the CS domain where a 2G/GERAN or 1XRTT (CDMA2000®) access network is employed, data services (including SMS) will likely degrade or suspend altogether.
Referring to FIG. 1, the EPS architecture for CWFB (100) showing the interconnection of a packet switched (PS) and CS domains for voice services and short message service (SMS) is illustrated as known in the prior art. As shown in FIG. 1, UE 110 has several radio frequency (RF) interfaces at its disposal, such as (i) a LTE-Uu radio interlace for connectivity between the UE 110 and an eNodeB (not shown) in the E-UTRAN access network 120; (ii) to Uu radio interface for connectivity between UE 110 and a NodeB (not shown) and RNC (not shown) in a legacy UTRAN access network 124, and (iii) a Um radio interface for connectivity between UE 110 and a legacy global system for mobile (GSM) communication/EDGE radio access network 122.
In order to allow the UE 110 access to GERAN 122 or UTRAN 124 for voice service via the CS domain, an SGs interface is deployed between the mobile switching centre (MSC) 142 and the mobile management entity (MME) 130. MSC 142 also acts as the gateway to fixed-line networks, such as a public switched telephone network (PSTN), and/or an integrated services digital network (ISDN) and the like. The S1-MME interface is provided for logical connectivity between UE 110 via access network (E-UTRAN 120) and the non-access stratum (NAS) via MME 130 as a function of the EPC procedures. UTRAN 124 connectivity with MSC 142 for the CS domain is provided by interface lu-cs. The serving GPRS support node (SGSN) 144 via the GS interface and the S3 interface provide connectivity to the PS domain for UTRAN 124 (by way of a serving gateway (S-GW)) and packet data network gateway (PDN-GW) not shown)). GERAN 122 connectivity with MSC 142 for the CS domain is provided by interface A, and limited PS domain access is provided via the Gb interface by way of SGSN 144.
SRVCC is a further function of EPS that allows VoLTE voice service in the PS domain to be handled by a legacy access network (e.g. GERAN/UTRAN/1xRTT). To that end, SRVCC provides for the transition of a voice call from the PS domain (VoIP/IMS) in the registered PLMN to a legacy access network operating in the CS domain. In SRVCC, the network and UE is required to be SRVCC-capable and engaged in a voice call. The SRVCC-capable UE determines that it is moving away from an E-UTRAN supporting VoLTE and notifies the network via the MSC which then moves the UE to the legacy access network in the CS domain.
Currently, if IMS (VoLTE), the SRVCC mechanism and the CSFB mechanism are not available due to various temporary network problems, a UE may be required to disable E-UTRAN network access to maintain only GERAN or UTRAN access to the network. In such a situation, the UE temporarily camped on the legacy access network cannot automatically reactivate E-UTRAN to access data services or benefit from such enhanced E-UTRAN features as IMS (VoLTE), the SRVCC mechanism and the CSFB mechanism (i.e. when in range of RATs that support these enhanced features). Alternatively, in a situation where a UE requests data services provided by the PS domain while in a voice call in the CS domain (e.g. IMS (VoLTE), the SRVCC mechanism and a CSFB mechanism are not currently available), there is no means of analysing connection requirements to determine if enhanced functionalities of E-UTRAN have become available upon movement of the UE.